Instrument suspension



March 19, 1957 Filed Aug. 17, 1953 R. E. H. CARPENTER INSTRUMENT SUSPENSION 2 Sheets-Sheet l Inventor Ma M 6M Attormy March 1957 R. E. H. CARPENTER 2,786,164

INSTRUMENT SUSPENSION Filed Aug. 17, 1955 2 Sheets-Sheet 2 Inventor I WEN. W

M aa/i z f A Horney United States Patent 2,786,164 INSTRUMENT SUSPENSION Rupert Evan Howard Carpenter, South Croydon, England Application August 17, 1953, Serial No. 374,743

Claims priority, application Great Britain August 27, 1952 8 Claims. (Cl. 317-198) This invention relates to instruments of the class having a defiectable pivoted member mounted on a spring suspension and, in particular, to electromagnetic transducers having a moving iron armature and especially to polarised relays coming within that category. In such electro-rnagnetic instruments, the armature experiences magnetic forces tending to produce unstable equilibrium and it is almost always necessary, in order to secure a reasonable degree of sensitivity or in order to make the armature when the relay is unenergised, maintain a central position, to use mechanical springs having positive stiffness partly or Wholly to balance out the negative stiffness produced by the magnetic forces. It is a fundamental requirement that the central position, towards which these springs tend to urge the armature, should be unaffected either by variations in temperature to which the apparatus is subjected, or by its preceding history as regards the position of the armature, that is the relay or other instrument should not exhibit bias effects due to creep in the springs.

Any variation in the position of this central positionthat is the position at which the net force exerted on the armature by the spring is zerowill appear as bias in the operation of the relay. in relays in which the mag netic negative stiffness is greater than the mechanical positive stiifness of the springs so that the armature is in unstable equilibrium and will remain on either of its stops if these be arranged on either side of the zero position, in short an each-side-stable relay, this bias gives rise to a change in the operating sensitivity of the relay with currents of opposite polarity, while in relays in which the stiffness of the spring is greater than the negative magnetic stiffness so that the armature tends to maintain a central positionthat is a centre-stable -relay not only is there the change in the different sensitivities mentioned above but the armature tends to drift from the central position and, in extreme cases, may touch one or the other of the side contacts in the absence of an operating current. v

The main object of the present invention is to eliminate as far as possible the change of bias with temperature. The invention also aims at reducing the effect of bias due to creep in the springs. A further object is to make it more easy in some cases to standardise the strength of the springs in the course of manufacture.

According to the present invention, in instruments having a part arranged to be capable of angular motion against the reaction of spring supports, a spring or springs is provided comprising a portion providing the major part of the compliance in the desired direction or manner-hereinafter called the working part of the spring and another portion bent at an angle to the first-mentioned portion, preferably a right angle, whose compliance in the direction or manner permitting the angular movement of the aforementioned part is small, but which has a willcient compliance in another direction or manner to permit a diiference in expansion due to temperature of the various parts of the apparatus to take place Without buckling or seriously distorting the first-mentioned part of the spring and thus permitting temperature changes to take 2,786,154 Patented Mar. 19, 1957 'ice ' 2 place without producing a bias on the moving part of the apparatus.

Such an arrangement by reducing the stresses due to differences of coefiicients of expansion also reduces any creep in all parts of the suspension. In order further to reduce flow or creep in the spring or springs under stress, the working part of the spring which operates in torsion may be made of smaller cross-section than the rest of the spring. Furthermore, the part of reduced crosssection which, of course, constitutes most of the working part of the spring may be made with the ratio of width to breadth near to unity since, in this manner, the maximum stress on any part of the cross-section of the spring is minimised. Clearly, the optimum cross-section for this part of the spring is a circle but as the production of such a spring with enlarged terminal pieces for attachment to the frame and to the moving part presents some difficulty in practice, it is sufiicient to make this part of the spring nearly square in cross-section or to keep the ratio of the dimensions at right angles to each other near to unity say, in any case, not more than 2 to 1. However, the necessity of giving the spring adequate lateral stiffness may, in practice, override these considerations.

The invention will now be more fully explained by describing an example of its application to a polarised electro-rnagnetic relay according to Patent No. 2,59,399, with reference to the accompanying drawings, in which:

Figure 1 is a front elevation of a polarised electromagnetic relay provided with a form of suspension in accordance with the invention;

Figure 2 is a front elevation to an enlarged scale of the two pole-pieces of the relay secured together with the armature and its spring mounting system;

Figures 3, 4 and 5 are respectively front elevation, side elevation from the left of Figure 3, and plan of one of the springs used in the suspension; and

Figure 6 is a side elevation similar to Figure 5 showing a slight modification.

Referring to Figmre l, a relay is shown having the features of that disclosed in Patent No. 2,559,399 and the same reference numerals are used for the corresponding parts. Thus, the two pole-pieces are seen at 1, 2. The relay coil is shown at '7, the permanent magnets at 9, the armature at 3 and the bars 6 are the supports for the fixed contacts. As-seen more clearly in Figure 2, the two pole-pieces l, 2 are identical and formed of magnetically soft alloy. As in the said prior specification, each consists of a flat plate with an arm 3 forming the pole-piece proper, a shank 4 and an upward projection 5 opposite the shank 4, each for carrying a bar which serves as the support for the fixed contacts of the relay as in the prior specification. The armature 13 is also similar and bears the moving contacts 20. However, the suspension strips shown in the drawings are modified into L-shaped form in accordance with the present invention and are seen at 14. Each strip consists of a horizontal working portion 21 which operates in torsion and a second portion 22 at right angles to the Working portion 21 and extending parallel to the edge surfaces of the armature 13.

Each L-shaped suspension strip is bent about a transverse axis which is parallel with the plane of the strip; in other words, it is bent about the axis of greatest compliance, so that the broad faces of sections 21 and 22 are arranged in planes at right angles to each other. The broad faces of portions 22 are clamped to the edge faces of the armature by clamping plates 23, 23a and bolts 24 which are threaded into the thicker plate 23 but pass with clearance through the armature 13. Instead, however, separate screws 24 could be used at either end both being threaded into the armature. The elbow .25 at the bend between the portions 21 and 22 is spaced from the end surface of the armature by providing the latter with a shallow recess or depression at 26 on either edge. The outer end of each working portion 271 is fixed to a mounting block 27 which itself is fixed to the polepiece 1 or 2 by a pair of screws 23 with a sheet 2) of insulating material interposed between the anchor block 27 and the pole-piece 1 or Z. The portion 21 is secured to the anchor block 27 by a screw 38 and a clamping plate 31. It will be seen that each L-shaped member 14 is mounted with its plane coincident with the central plane of the armature, whereby any movement of the armature due to differential expansion between the armature assembly and the frame takes place in a direction parallel with the central plane of the armature and does not disturb the spacing between the fixed and movable contacts.

In Figure 6, a somewhat modified form of the spring strip 14 is shown in that a slot 32 is punched out thereby rendering the strip more flexible or increasing its compliance in a direction parallel with the plane of the armature 13.

As best seen in Figure 4, the greater part of the working portion 21 of the spring strip 14 is reduced in width over an accurately predetermined length. it will then be realised that if the sum of the expansions of the springs 14 and of the armature 13 is different from that of the pole-pieces 3 between the two fixed anchor blocks 27, this difference can be taken up at the elbows 25 in the springs which are free to move closer to or further from the armature 13 without unduly stressing or distorting the springs 14. During this process the portions 22 of the springs act as cantilevers allowing the elbows 25 to yield relatively freely.

In the assembly of such a relay, most careful precautions have to be taken to ensure that the armature 13 is set centrally and parallel in the working gap between the pole-pieces 3. This working gap is established by spacers between the pole-pieces 3 in the form of sleeves surrounding screws passing through holes 32 in the pole-pieces 3 exactly as in the prior specification already referred to.

It is convenient to assemble the armature 13 and the suspension springs 1 in an assembly fixture or jig provided with plane surfaces accurately at right angles to one another, against which the working face of the armature 13 and the faces of the suspension springs 14 are pressed before the screws 24 are tightened. it. is also necessary, of course, to ensure that the springs 14 are so curately spaced in the vertical direction as well as angularly with respect to the armature 13 before the screws 24 are tightened.

The springs 14 may be made of beryllium copper, and may have a thickness, for example of 0.015 inch. This matc'ial is capable of precipitation hardening so that after bending a sprint at 25 it can be rehardened in a suitable fixture to assume the desired shape and to establish maximum stability in it.

Clearly, the arrangement may be somewhat modified, for example one end of the spring may be directly fixed to the armature as in Figure 4 of the drawings of Patent No. 2,559,399. and the working portion ex ends at right angles to the armature, and a part at the outer end of this horizontal portion be turned at a right angle and then fixed by screws to side surfaces of anchor blocks, such as blocks 27. Then the elbows in the spring strips which float to take up the difference in expansion lie at the ends of the working portions 21 near to the anchoring blocks 27.

I claim:

1. In an electromagnetic instrument having a movingiron armature pivoted in the stationary frame of the instrument, a pair of spring strips forming a suspension system for said armature, each strip being secured near one end thereof respectively to opposite parallel edge surfaces of said armature and secured near the opposite end thereof to the frame of the instrument, each of said spring strips being formed with a right-angle bend in its length to provide to one side of the bend, a working portion stressed in torsion to exert the restoring force for said armature and, to the other side of the bend, a second portion providing a floating elbow at the region of the bend.

2. An electromagnetic instrument according to claim 1, wherein the working portion of each of said spring strips lies in a plane containing the pivotal axis of said armature and the second portion of each of said spring strips lies substantially parallel to the edge surface of said armature to which it is secured.

3. An electromagnetic instrument according to claim 1, wherein the working portion of each of said spring strips is made for the greater part of its length of smaller cross-section than the remainder of said respective spring strip.

4-. An electromagnetic instrument according to claim 2, wherein the working portion of each of said spring strips is made for the greater part of its length of smaller cross-section than the remainder of said respective spring strip.

5. An electromagnetic instrument according to claim 4, wherein the part of smaller cross-section of each of said spring strips has the ratio of the dimensions of its cross-section at right angles to each other not greater than 2:1.

6. An electromagnetic instrument comprising a supporting frame, an armature, mounting means supporting said armature upon said frame for pivotal movement about a predetermined axis comprising a pair of L shaped spring members arranged on opposite sides of said armature, one end of each spring member being connected with the armature and the other end being connected with said frame, said spring members being an ranged with one arm of each member extending along said axis and providing the greater part of the compliance permitting angular vibration of the armature about said axis, and the other arm of each spring member extending parallel with the axis of said armature and providing compliance permitting differential thermal expansion of the armature assembly and the frame with low stress on said spring members.

7. An electromagnetic instrument comprising a supporting frame, an armature, mounting means supporting said armature upon said frame for pivotal mGJC- mcnt about an axis located within the central plane of the armature comprising a pair of L-shaped spring members arranged on opposite sides of said armature and having their planes coinciding with said central plane, one end of each spring member being connected with the armature and the other end being connected with said frame, said spring members being arranged with one arm of each member extending along said axis and providing the greater part of the compliance permitting angu lar vibration of the armature about sai axis, and the other arm of each spring member extending parallel with the axis of said armature and providing a compliance permitting differential thermal expansion of the armature assembly and the frame with low stress on said spring members.

8. An instrument according to claim 6 wherein said two spring members have the same configurations and the same dimensions on opposite sides of a central plane of the armature, whereby movement of the armature due to differential thermal expansion between the mature assembly and the frame, takes place a direction parallel with the central plane of the armature.

References Cited in the file of this patent UNITED STATES PATENTS 

